1. Field of the Invention
The present invention relates generally to mounting devices of the nature used to support vibrating bodies, and more specifically to vibration damping mounting device which is compact and particularly suited to automotive applications.
2. Description of the Prior Art
JP-A-60-104828 describes an example of damping type unit which takes the form of a hollow elastomeric bush member filled with an electrorheological fluid (or ERF as it will be referred to hereinafter) and which is further provided with an electrically controlled orifice unit which divides the device into a working chamber and an expansion chamber. With this device, when the bush is compressed, fluid is displaced from the working chamber into the auxiliary one (defined between the orifice unit and a relative flexible diaphragm) and vice verso. By applying a high voltage across the electrodes of the orifice, the viscosity of the ERF can be induced to undergo a remarkable increase and the fluid in the orfice passage induced to assume to an almost solid state. Under these conditions the orifice is either effectively blocked or the restrictive properties thereof remarkably increased.
When this type of arrangement is used to support internal combustion engines, for example, it is possible to selectively tune the characteristics of the device in manner which improves the effective vibration damping of the system defined by the engine, mounts and chassis on which the engine is supported. However, in order to effectively damp relatively large amplitude low frequencies (10-30 Hz), such as occur when the engine is idling or undergoes what is referred to as "engine shake", the device becomes excessively bulky and induces design problems.
Experiments conducted by the applicant in connection with this problem, have revealed that if the above type of arrangement is arranged to act as a dynamic damper type arrangement the resonance frequency fo of the mass of the fluid in the orifice passage(s) can be expressed using the following equation: ##EQU1## wherein: S1: is the cross sectional area of the orifice;
S2: is the cross sectional area of the fluid chamber; PA1 m: is the mass of the slug of fluid in the orifice; and PA1 k: is the expansive spring constant of the fluid chamber.
As will be noted from this equation, in order to tune the arrangement so that the resonance frequency occurs in a low frequency range, it is necessary to either necessary to increase the value of m or reduce S1.
It should be noted that m=orifice length.times.specific gravity of the fluid.
As will be appreciated from the broken lines in FIG. 9, in the event that the orifice cross-sectional area S1 is reduced, the flow resistance of the same increases whereby, in the low frequency region below that at which resonance of the fluid in the orifice occurs, the dynamic spring constant assumes a high value and while the loss factor assumes a low one. This of course is the reverse of the conditions required for appropriate vibration attenuation and thus results in loss of effective vibration damping.
Hence, a solution to the bulkiness of the above mentioned prior art type arrangement has been difficult.